Translation:Por favor firma.
Commands are handled differently. Generally speaking, you use the present subjunctive for commands. However, for an affirmative (yes) informal (tu) command, you use the usted form of the present indicative form.
That was confusing, I know. Read this: http://www.studyspanish.com/lessons/informcomm1.htm
To add, if you were speaking informally, the translation would "Firma (tú), por favor", for reasons stated above. If you were speaking formally, the translation would be "Firme (Ud.), por favor" (http://www.studyspanish.com/lessons/formcomm.htm). Basically, there's different verb conjugations for commands based on if it's positive ("Do this'), negative ("Don't do this"), formal ("Usted"), or informal ('tú"). You can see more examples of this on the websites listed above. Hope this clarifies further!
Oh wow, you guys are way over thinking it! The best way to learn any language is immersion (movies, music, books, articles; a fully Spanish-speaking environment, etc) The oldest trick in the book! And guess what, when you go abroad nobody is going to care whether you use the right conjugation or not. 80-90% of communication is body language and facial expressions, so as long as you use the correct basic verb, they'll get what you're saying. And they will be happy to correct you if you ask them to. Not to mention that if you immerse yourself enough, after a while you'll just "know" the right conjugation without having to memorize anything, because it'll "feel right." Have more faith in your brains people. It'll do the right thing! (:
Yeah, dirty trick to give us infinitive and expect us to realize we needed to conjugate it, before we learned the 3 infinitives and how they differ :p